Method and apparatus for monitoring a lubricant

ABSTRACT

An apparatus for monitoring a lubricant in a machine comprises sensors S1, S2 for producing signals representing electrostatic activity in a machine lubricant and a temperature sensor for producing a signal representative of the temperature of the lubricant. A signal processor 9 is arranged to process the signals from the sensors S1, S2 to detect an electrostatic activity precursor indicative of an impending wear event in the machine. The signal processor is arranged to operate in response to the temperature sensor to compensate for temperature related changes to the signals from the electrostatic sensors S1, S2.

FIELD OF THE INVENTION

The invention relates to an apparatus for and method of monitoringlubrication. The invention can be applied to the monitoring oflubrication in a machine for example to enable advance detection of wearin the machine. As used herein the term "machine" is intended to includeengines, gearboxes and other mechanical systems in which fluidlubrication is used.

BACKGROUND OF THE INVENTION

In International Patent Application No. PCT/GB 91/02112 published as WO92/09886 there is described a system for monitoring debris in a fluid.The system comprises at least one electrostatic sensor for producing asignal representing electrostatic charge associated with the fluidmoving past the sensor and with debris and/or impurities carried by thefluid. The signal from the sensor is conditioned by a signal conditionerand the conditioned signal is processed by a signal processor togetherwith another signal representing at least the charge associated with themoving fluid in order to produce a signal representing the electrostaticcharge associated with the debris and/or impurities. In practice twoelectrostatic sensors are used at spaced apart locations and the signalstherefrom are processed, i.e. correlated, in order to identify theexistence of debris in the moving fluid. The system is extremely usefulfor detecting wear in machines because machine wear causes chargecarrying particles to be generated which can be detected by the system.

SUMMARY OF THE INVENTION

In one aspect the invention provides an apparatus for monitoring alubricant in a machine, the apparatus comprising a sensor for producinga signal representing electrostatic activity in a machine lubricant, anda signal processor for processing the signal from the sensor to detectan electrostatic activity precursor indicative of an impending wearevent in the machine.

In another aspect the invention provides a method of monitoring alubricant in a machine, the method comprising producing a signalrepresenting electrostatic activity in a machine lubricant, andprocessing the signal to detect an electrostatic activity precursorindicative of an impending wear event in the machine.

In another aspect the invention provides a method of and system foranticipating wear in a machine by monitoring the lubricant thereof foran electrostatic activity precursor indicative of an impending wearevent.

In another aspect the invention provides a method of and system formonitoring a machine lubricating fluid, in which method and system theactivity level of an electrostatic signal is monitored for a changewhich is interpreted as a precursor to an electrostatic eventcorresponding to a wear event.

The detection of a precursor may be utilised to activate an indicator inadvance of the wear/electrostatic even to enable corrective action to betaken to avoid the event. Thus, wear or even catastrophic failure in amachine may be avoided as a result of the advance warning enabled by theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will be described hereinafterwith reference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a lubrication monitoring system; and

FIG. 2 shows signals derived from various sensors in the system of FIG.1.

Turning now to FIG. 1 of the accompanying drawings there is shown agearbox 1 with an associated oil pump 2 connected to the gearbox 1 byway of a conduit 3. The conduit 3 provides a path for oil from thegearbox to the pump 2 and from the pump to the gearbox. Two sensors S1and S2 are mounted to the conduit 3 some distance apart from each other.The sensors S1, S2 are provided for sensing electrostatic charge and anysuitable form of sensor may be used. Electrical cables 5, 6 from thesensors S1, S2 are connected to a signal conditioner 7. The signalconditioner 7 is provided to perform a preliminary conditioning of thesignals from the sensors S1, S2. The signals from the sensors S1, S2 areinduced by electrostatic charge passing the sensors. As such the signalstend to be weak, one purpose of the signal conditioner 7 is to giverobustness to the signals. The signal conditioner 7 may also act as acoarse filter to remove noise from the signals and may even includeintegrating and differentiating circuits if so required. Signalconditioning and signal conditioning circuits are per se well known andwill not be described in any greater detail herein.

Conditioned signals from the signal conditioner 7 are recorded by a taperecorder 8 for subsequent analysis by a signal processing circuit, i.e.a computer 9. Alternatively, or additionally, the signal processing maybe performed by the signal processing circuit 9 on-line and in realtime. Results of the signal processing can be displayed on a display 10in any suitable form for inspection by a user. An oscilloscope 11 mayalso be connected to the tape recorder 8 to allow a user to inspect theconditioned signals from the signal conditioner 7 prior to processing bythe signal processing computer 9. Among other things the signalprocessing computer 9 processes the signals to determine theelectrostatic activity level represented by the signals. The activitylevel is determined by calculating a rolling RMS value from the signals.Alternatively or additionally such methods as high and/or low passfiltering and/or Fast Fourier Transforms (FFTs) may be used by thesignal processing computer to determine the electrostatic activity levelin the lubricant as represented by the signals.

As oil flows through the conduit 3 electrostatic charge in the oilinduces a signal onto each of the sensors S1, S2. Even with no debrispresent in the oil there will be a background signal induced byelectrostatic charge generated in a similar manner and carried by theoil and from noise sources such as air bubbles and the like introducedby aeration of the oil. The signal conditioner 7 may include a high passfilter for filtering out low frequency noise from such noise sourcesdetected by the sensors S1 and S2. The flow rate of the oil is known andit is also possible to use that knowledge to process the signals fromthe sensors to remove further noise therefrom.

The electrostatic charge and therefore the electrostatic signal varywith temperature of the lubricant. This temperature effect iscompensated for by monitoring the lubricant temperature and theelectrostatic signal when the lubricant is "clean", i.e. does notcontain debris or other impurities. The temperature effect isrepeatable, and the variation as temperature changes can therefore bepredicted. The prediction is used as a baseline datum against whichelectrostatic and temperature signals are checked. The signal processingcomputer 9 is arranged to monitor for changes in the electrostaticsignal which are different than those predicted in the baseline datum(i.e. different than those predicted and expected in advance against agiven change in temperature).

Noise reduction techniques are used to increase the signal to noiseratio of the signals from the sensors. Further noise cancelling andsignal processing techniques are described in WO 92/09886, the teachingsof which are incorporated herein by reference.

The system shown in FIG. 1 further comprises a third sensor S3 with anassociated microcontroller 12 for detecting debris in the oil in theconduit. The third sensor S3 is not necessary to put the invention intoeffect, it is merely included to provide a reference against whichprinciples behind the invention will be explained. In tests acommercially available debris monitoring system was used to senseferrous debris in the oil and it is the results of those tests that areshown in FIG. 2 of the accompanying drawings. The sensor S3 istemperature sensitive and the system therefore comprises a thermocoupleto enable temperature corrections to be made. The commercially availablesystem will not be described in any further detail herein.

In tests, an FZG lubricant testing machine was used as the gearbox 1.The FZG lubricant testing machine comprises gears which are configurableenable wear particles to be generated under known conditions, forexample scuffing of the gears.

FIG. 2 of the drawings shows conditioned signals from the sensors S1 toS3 and a temperature signal from the thermocouple 14 obtained during atest run. As can be seen from FIG. 2, the test run lasted for some 3000seconds (50 minutes) during which time conditions within the gearboxwere varied. The conditioned signals from the electrostatic sensors S1and S2 are represented by the dotted line 20, one conditioned outputsignal from the commercially available system sensor is represented bythe line 21 and the temperature signal from the thermocouple isrepresented by the line 22.

The test gearbox was run under approximately steady conditions for about1400 seconds to allow the gearbox to reach normal running temperatures.The test gearbox includes a cooling system (not shown) and after about1400 seconds that was turned off. Then at about 1600 seconds a heater(not shown) was switched on to increase the operating temperature of theoil. At around 2200 seconds the speed of the gearbox was doubled andshortly thereafter a wear event occurred resulting in the generation ofdebris. The debris was detected by system sensor S3 as can be seen bythe spike 23 in the line 21. The debris was also detected by theelectrostatic sensors S1 and S2 as can be seen by the peak 24 in theline 20. The peak 24 is an electrostatic event and occurs before thespike 23 because, among other reasons, the sensors S1 and S2 areupstream of the sensor S3.

In the period between the heater being switched on and the engine speedbeing doubled (at which time the wear event occurred) there is a smallbut steady increase in the level of the signal 21 from the system sensorS3. It will be noted that this increase corresponds with the increase intemperature and is therefore a characteristic of the sensor rather thanan indication of increased debris in the oil. A temperature-correctedsignal from the system sensor would not exhibit such an increase. Untilthe wear event occurs at approximately 2300 seconds there issubstantially no increase in debris in the oil.

In the same period there is a significant increase in the conditionedsignal from the electrostatic sensors S1 and S2. These sensors are nottemperature sensitive but the increase in signal level is greater thanwould be expected in advance from the pre-calculated baseline datum. Theincrease in signal level therefore corresponds to an increase inelectrostatic activity within the oil. The increase in the sensedelectrostatic activity is caused by an increase in physical/chemicalreactions within the oil as a result of increased loadings of the oil inthe gearbox.

Many lubricants, including the oil used during the test, contain anextreme pressure (EP) additive. The nature of EP additives is to adherefirmly to the surface of the metal of the gears in the gearbox. EPadditives remain largely inactive until high contact temperatures arereached, typically 600-800° C. These are not bulk temperatures but occurwhen asperities interact. That is to say there will be local "hot-spots"on the gears and it is at those locations that the EP additives becomeactive. At these high temperatures the EP additive moleculedisassociates, producing chemically very active charged species whichattack the high temperature metal creating an oxide type film whichhelps prevent contact welding (scuffing).

The electrostatic sensors detect the increase in charge species in theoil stream created by the EP chemical action; particles of the oxidefilm and byproducts of the dissociation are relatively highly chargedbecause the oxide films are insulators and rubbing them together willcause electrostatic charge to be generated.

When the contact becomes more extreme with the doubling of the speed,the EP additive on the surface of the gear is overwhelmed and itscontribution to the charge in the oil diminishes. Any metal-metalcontact would then lead to contact welding, hence scuffing whichgenerates particles, i.e. wear events, that are detected aselectrostatic events by the electrostatic sensors and as signal spikesby the sensor in the usual manner.

The initial full metal-metal contact also provides an earthing route,quenching the charge level. This is another reason why the activitylevel of the electrostatic sensor signal falls just as the signal startsto indicate the presence of wear particles. However, it should be notedthat the electrostatic sensor signal remains higher than earlier in thetest. Also the signal is increased in amplitude compared to what wouldbe predicted from a baseline datum at the appropriate oil temperature.

Should the wear become severe, then the bulk metal temperatures, i.e.the temperature of the gears in the gearbox, will rise rapidly,oxidising the oil and metal surface and creating charge-carrying carbonparticles which are detectable by the electrostatic sensors. In apreliminary step-loaded scuffing test, where mild scuffing was notencountered, the heavy scuffing and over-heating was accompanied by highcharge in the oil.

It will be appreciated from the foregoing that the use of anelectrostatic sensor enables an increase in electrostatic activityresulting from increased loading or other adverse conditions to bedetected. Such an increase in electrostatic activity is a precursor to awear event and occurs as a result of physical/chemical reactions betweenthe lubricant and the machine.

In the above discussed example there is a significant increase in thesensor signal before the wear event occurs. In some situations theprecursor may not be represented by such a large change. The signalprocessing computer 9 may be arranged to increase the signal to noiseratio and emphasise the presence of the precursor by way of the abovementioned rolling RMS value, high and low pass filtering and FFT signalprocessing techniques for example. The operation of many machines iscyclic and advantage can be taken of this by the signal processingcomputer 9 in the use of said rolling RMS values, high and low passfilterings and FFTs to identify an event precursor in a signal.

Monitoring the electrostatic signal for a wear event precursor enablesaction to be taken before the event to avoid wear of the machine. Suchavoiding action may simply be a reduction in operating speed/loading ofthe machine, or might involve an oil change either while the machine isrunning (if possible) or with the engine switched off. It can be seenfrom FIG. 2 that the precursor is detectable several minutes before thewear event. Even in an aircraft where engine shut-down was necessarythis would give the pilot sufficient time to land safely withoutdamaging the engine.

Having thus described the present invention by reference to a preferredembodiment it is to be well understood that the embodiment in questionis exemplary only and that modifications and variations such as willoccur to those possessed of appropriate knowledge and skills may be madewithout departure from the spirit and scope of the invention andequivalents thereof.

We claim:
 1. An apparatus for detecting an impending wear event in amachine before its actual onset by monitoring a lubricant in a machine,the apparatus comprising a sensor for producing a signal representingelectrostatic activity in a machine lubricant, and a signal processorfor processing the signal from the sensor to detect an electrostaticactivity precursor indicative of an impending wear event in the machine.2. An apparatus as claimed in claim 1, wherein the signal processor isarranged to process the signal from the sensor to detect a change in thesignal as representing the electrostatic activity precursor.
 3. Anapparatus as claimed in claim 1, wherein the signal processor isarranged to process the signal from the sensor to detect a steadyincrease in the amplitude of the signal as representing theelectrostatic activity precursor.
 4. An apparatus as claimed in claim 1,further comprising a signal conditioning circuit for conditioningsignals from the sensor.
 5. An apparatus as claimed in claim 4, whereinthe signal conditioning circuit is connected to output conditionedsignals to the signal processor.
 6. An apparatus as claimed in claim 4,further comprising a recorder for recording signals from the sensorand/or from the signal conditioning circuit.
 7. An apparatus as claimedin claim 6, further comprising an oscilloscope for displaying signalsfrom the sensor, the signal conditioning circuit and/or the recorder. 8.An apparatus as claimed in claim 1, wherein the signal processor isoperable in response to the detection of an electrostatic activityprecursor to activate an indicator.
 9. An apparatus as claimed in claim1, wherein the sensor comprises a first electrostatic sensor for sensingelectrostatics in lubricant at a first position in a machine and asecond electrostatic sensor for sensing electrostatics at a secondposition in the said machine.
 10. An apparatus as claimed in claim 1,wherein the sensor further comprises a temperature sensor, and thesignal processor is operable in response to the temperature sensor forprocessing the signal representing electrostatic activity to compensatefor temperature related changes thereto.
 11. A method of detecting animpending wear event in a machine before its actual onset by monitoringa lubricant in a machine, the method comprising producing a signalrepresenting electrostatic activity in a machine lubricant, andprocessing the signal to detect an electrostatic activity precursorindicative of an impending wear event in the machine.
 12. A method asclaimed in claim 11, wherein the signal is processed to detect a changetherein as representing the electrostatic activity precursor.
 13. Amethod as claimed in claim 11, wherein the signal is processed to detecta steady increase in the amplitude thereof as representing theelectrostatic activity precursor.
 14. A method as claimed in claim 11,further comprising conditioning the signal prior to said processing. 15.A method as claimed in claim 14, further comprising recording the signaland/or the conditioned signal.
 16. A method as claimed in claim 15,further comprising displaying the signal, the conditioned signal, therecorded signal and/or the recorded conditioned signal.
 17. A method asclaimed in claim 11, further comprising sensing electrostatics inlubricant at a first position in a machine and sensing electrostatics inthe lubricant at a second position in the said machine.
 18. A method asclaimed in claim 11, further comprising processing the signalrepresenting electrostatic activity to compensate for temperaturerelated changes thereto.
 19. A method of detecting an impending wearevent in a machine before its actual onset by monitoring a machinelubricating fluid, in which method the activity level of anelectrostatic signal is monitored for a change which is interpreted as aprecursor to an electrostatic event corresponding to an impending wearevent.
 20. A system for detecting an impending wear event in a machinebefore its actual onset by monitoring a machine lubricating fluid, inwhich system the activity level of an electrostatic signal is monitoredfor a change which is interpreted as a precursor to an electrostaticevent corresponding to an impending wear event.